linux-kernel - Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Message-ID: <e3d2dfaa-9f33-85b5-38cc-2ded315fe993@linux.intel.com>
Date:   Tue, 9 Jan 2018 09:55:30 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On 01/09/2018 02:40 AM, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Peter Zijlstra wrote:
>> On Mon, Jan 08, 2018 at 09:28:12AM -0800, Tim Chen wrote:
>>> On 01/08/2018 08:14 AM, Peter Zijlstra wrote:
>>>> On Mon, Jan 08, 2018 at 01:47:07PM +0100, Peter Zijlstra wrote:
>>>>>> a good suggestion, but we encountered some issues with it either
>>>>>> crashing the kernel at boot or not properly turning on/off.
>>>>
>>>> The below boots, but I lack stuff to test the enabling.
>>>
>>> Peter,
>>>
>>> Thanks.  Will give it a spin.  One other concern is if
>>> JUMP_LABEL is not configured, this may not work, and
>>> we may still need fall back to a control variable.
>>
>> Urgh yes.. I always forget about that case. Will the retpoline crud be
>> backported to a GCC old enough to not have asm-goto? If not, we could
>> make all of this simply require asm-goto.
> 
> No, ifs and buts, really.
> 
> This wants to be a jump label and we set the requirements here. I'm tired
> of this completely bogus crap just to support some archaic version of GCC.
> 
> This is messy enough and no, this whole we need a control variable nonsense
> is just not going to happen.
> 
> 

Thomas,

I'll be sending an updated patchset with boot option opt in for ibrs
and leave the control varaible out.  I agree that we can worry about the
control variable later.

Thanks.

Tim